22,3 Reyes: Woods of the Philippine Dipterocarps 309 



In sectional view the simple pit is seen (Plate 9, fig. 12) as 

 a thin place in the cell wall arising through the absence of 

 secondary layers or opposite sides of the middle lamella. The 

 secondary layers end abruptly (without arching over) causing 

 a break in the cell wall which is spanned only by the middle 

 lamella. The structural details of the simple pits of Parashorea 

 exhibit no departures from the usual type. 



The bordered pits, on the contrary, present a wide range of 

 variation in conformity with that of the elements on which 

 they occur. Those that are found on vessel or tracheid walls 

 are rounded or oval in surface view with a circular or flattened 

 orifice. Such elements, while performing the mechanical func- 

 tion, are primarily concerned in water conduction and elongate 

 but little following their origin in the lateral meristem. As 

 a result, the pits are not stretched to any great extent. Fiber 

 tracheids and libriform fibers, on the other hand, undergo 

 marked changes as they mature, especially in longitudinal 

 dimension, to fit them for their mechanical role. Consequently 

 the bordered pits become very much flattened and attenuated 

 spirally, and the pit orifice is reduced to a mere slit. The pits 

 may become so altered in libriform fibers as to appear simple 

 (Plate 9, fig. 9). 



Plate 9 (figs. 1 to 9), presents various aspects of bordered or 

 semibordered pits in sectional view, and the several departures 

 deserve mention here. Pits leading from tracheid to tracheid 

 are of the type common in coniferous and dicotyledonous wood 

 except for the absence of tori. The pit cavity is widest at the 

 middle lamella and tapers gradually toward the cell lumina. 

 A curious modification, however, is to be noted in the bordered 

 pits of vessel walls, since in these the pit flares again at either 

 orifice giving the appearance of a double dumb-bell, a condition 

 that is observable only in cross section; in longitudinal section 

 such pits appear as in Plate 9, fig. 3, indicating that the con- 

 striction in the pit cavity is not annular but is limited to the 

 sides. It follows that where vessels abut on tracheids the pit 

 cavity is constricted on the vessel side, but of normal contour 

 in the tracheid wall. Figures 7 to 9 of this plate are sections of 

 pits that are very much attenuated, a type common on the walls 

 of libriform fibers. 



In conclusion, mention must be made of the intercellular 

 spaces which are characteristic features of all woody tissue 

 and provide a means of aeration for the cells that retain living 



